Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the place where the tire has lost its pressure to a place desired by the driver, such as a service station, where the tire can be changed. Loss of tire pressure can, of course, result from a variety of causes, including puncture by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for sustained operation under conditions of no pressure or underpressurization are also called runflat tires, as they are capable of being driven, or “running,” while uninflated, or in what is generally called a “flat” condition.
The conventional pneumatic tire, when uninflated, collapses upon itself when it is carrying the wheel loading of a vehicle. The tire's sidewalls buckle outward in the region where the tread contacts the ground, making the tire “flat.”
Runflat tires are also called extended mobility tires, the latter phrase being compressed to EMT, which is a term of art. A tire designed for runflat or EMT operation is designed such that the tire structure alone, and in particular the structure of the sidewalls, has sufficient strength and rigidity to support the vehicle's wheel load and provide adequate vehicle handling when the tire is not inflated. The sidewalls and internal surfaces of EMT tires do not collapse or buckle onto themselves. Such tires do not typically contain or use other supporting structures or devices to prevent the collapse into a flattened condition due to loss of tire pressure.
Among the many examples of runflat tires is a tire described in U.S. Pat. No. 4,111,249, entitled the “Banded Tire,” in which a hoop or annular band, approximately as wide as the tread, is circumferentially deployed beneath the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition.
Another approach toward EMT or runflat technology has been simply to strengthen the sidewalls by increasing their cross-sectional thickness. That is, runflat tires incorporate sidewalls that are thicker and/or stiffer so that the tire's load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. However, due to the large amounts of rubber required to stiffen the sidewall members, cyclical flexural heating during runflat operation becomes a major factor in tire failure. This is especially so when the uninflated tire is operated for high speeds and the cyclical flexure rate is correspondingly high. Pirelli discloses an example of a runflat tire design having thickened sidewalls in European Pat. Pub. No. 0-475-258A1.
Typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed “wedge inserts” within the concave inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. The wedge inserts, which are often referred to simply as inserts, are usually crescent shaped in cross-sectional views. The incorporation of one or more wedge inserts within each sidewall of a tire thickens the sidewalls substantially. Sidewall thickness in the region between the bead and the tread shoulder is often nearly uniform, when viewed in cross section.
U.S. Pat. No. 5,368,082, by Oare et al, having a common assignee, discloses a low-aspect-ratio, runflat pneumatic radial ply tire which employs wedge inserts in the sidewalls. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. This invention, although superior to prior attempts at runflat tire design, imposed a weight penalty. Fortunately, that weight penalty could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios and in tires designed for high performance.
U.S. Pat. Nos. 5,427,166 and 5,511,599 to Walter L. Willard, Jr., disclose the addition of a third ply and the addition of a third insert in the sidewall of a runflat tire to theoretically further increase the runflat performance of the tire over that of the U.S. Pat. No. 5,368,182. The Willard patents discuss some of the load relationships that occur in the uninflated condition of the tire and demonstrate that the Oare et al. concept can be applied to additional numbers of plies and inserts.
In general, runflat tire design is predicated upon the installation of reinforcing inserts inside each sidewall flex area. The inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. The U.S. Pat. No. 5,368,082 teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall has drawbacks which include the above mentioned increase in tire weight and flexure-induced heat build up. Such designs also increase the tire's complexity in ways that adversely affect manufacturing operations and quality control.
U.S. Pat. No. 4,287,924, by Deck et al, places a two part reinforcing wedge inside the carcass which is described as: “a support member (20) of lenticular section, made of an elastomer and extending from the vicinity of the beads (12) to below the edges of the belt . . . [and] constituted by two parts with different flexibility . . . . ” In order to promote evacuation of the heat produced in the thick part of the supporting shaped parts (20), means are provided which consist of a heat-conducting sheet or layer (28) embedded in the body of shaped parts (20) and extending at least to the vicinity of beads (12). Preferably the layer extends over the whole height of shaped parts (20), from the upper part of the side walls adjacent to the belt of crown (23) to the vicinity of the beads. The layer (28) is advantageously disposed between the two parts of shaped part (20), and can be constituted for example by flexible parallel metallic cords extending along the radial planes of the tire.
European Patent Application No. EP 0 822 105 A2, by Walter et al (Michelin), comprises a plurality of crescent-shaped reinforcing members (5) and a plurality of carcass layers (6), and provides first and second partial carcass layers (62, 64) along with a filler rubber portion (24) and a second crescent-shaped reinforcing member (56). In the Walter application's FIG. 4, it can be determined that the placement of elements in the sidewall, from the inside out, is: innerliner (44), first crescent-shaped reinforcing member (54), inner carcass layer (68), second crescent-shaped reinforcing member (56), first partial carcass layer (62), filler rubber portion (24), second partial carcass layer (64), and outer sidewall (42). The filler rubber portion is placed above the bead similar to an apex. FIG. 6 shows a variation with a different shaped filler rubber portion (24a), but still places the wedges (54, 56) on either side of the inner carcass layer, and places the partial carcass layers (62, 64) axially outside of the [complete] carcass layer (68). The partial carcass layers extend from the vicinity of the bead up under the crown region to a specified distance (L1) axially inside of the edge of the tread belt. As best it can be determined, the partial carcass layers are comprised of standard carcass ply materials.
In summary, the goals in runflat tire design are to provide a low-cost, light-weight tires that give good runflat vehicle handling as well as good service life during runflat operation. Accordingly, the considerations that must be taken into account in runflat tire design include the use of minimal additional material to the tire (to achieve the desired sidewall stiffness) as well as minimal additional steps in the manufacturing process.
Among the design considerations for EMT or runflat technology is the nature of the sidewall loading and corresponding stresses that arise during runflat operation. For example, during runflat operation, the insert-reinforced sidewalls still undergo an outward or axial buckling (though not to the extreme extent of an uninflated non-runflat tire) in the region adjacent to where the tire's tread makes contact with the road. Such “loading” of the sidewalls causes them to deform in such a way that the outermost portions of the sidewalls, including especially the outermost carcass plies, experience tensile stresses. At the same time, the innermost portions of the insert-reinforced sidewalls, e.g., the portions of each wedge insert that lie closest to the tire's innerliner, experience large increases in compressive stresses. This combination of tensile and compressive stresses is also cyclical as the tire rotates. It follows that as the vehicle moves faster, the rate of flexural heat buildup increases.